The electronic article surveillance (EAS) industry has looked at large to tag devices of a type involving a dipole antenna housed with a diode in a protective envelope of insulative material. In some instances, EAS systems have provided for the transmission of a high frequency signal, such as a 915 megahertz carrier, and of a lower frequency signal, such as modulated 100 kilohertz. Widespread understanding, as evidenced in Pinneo et al. U.S. Pat. No. 4,413,254, is that such device defines a so-called "receptor-reradiator", returning to the receiver of the EAS system, the 915 MHz carrier with content related to the lower frequency and its modulation characteristic. Upon detection in the receiver of received signals inclusive of the modulation characteristic in given repetitive succession, an alarm indication is provided.
Generally, detection takes place in a controlled zone, i.e., an exit area of a retail establishment, and output alarm indication is that of a tag device being carried therethrough without authorization (undeactivated).
The art has come to realize substantial analytical evaluation of the activity at hand in EAS dipole and diode tag devices. Thus, in Woolsey et al. U.S. Pat. No. 4,642,640, there is a recognition of the need to establish circuit parameters which maximize the reception of the various signals transmitted, the need for establishing an inductive tag device character at the high frequency, where length parameters otherwise dictate, and the need of having a resonant circuit in the tag device at the high frequency.
In addressing such discerned needs, Woolsey et al. looked to the addition of inductance at 915 MHz selectively, as by a serpentine inductive path providing same within the length constraint at hand. Woolsey et al. thus looked not to the simple dipole/diode combination but to a discernment of specific diversely characterized tag device areas. They provided a generally rectangular tag configuration, devoting area to a circuit element which is inductive at the high frequency and is capacitive up to the lower frequency and other area to another circuit element, which is inductive at the high frequency, the circuit elements being physically disparate in geometry and arranged in electrical series circuit with the diode. There was a particular recognition that the sum of the various reactances of the circuit elements and that of the diode should give rise to situations wherein the diode is at the center of a resonant circuit, wherein the net sum of the various reactances at hand across the tag should then be zero and wherein the circuit elements should be addressed generally to different purposes, e.g., that one thereof should be such as to maximize second lower frequency energy receipt and hence voltage applied to the diode.
A further advance in the type of tag device under discussion is seen in Siikarla et al. U.S. Pat. No. 4,736,207 to which incorporating reference is hereby made. In its preferred form, the Siikarla et al. tag device is of generally rectangular configuration and comprises a first circuit element extending longitudinally of the device and of first transverse dimension, a second circuit element extending longitudinally of the device at least in part jointly with the first circuit element and of second transverse dimension substantially exceeding the first transverse dimension and effecting predominant different receipt by the first and second circuit elements of the high and low frequency transmitted signals and a further circuit element exhibiting voltage dependent capacitive reactance connected in electrical series circuit with the first and second circuit elements.
The third circuit element, which is typically a diode, has applied thereto the voltage generated in the tag device in response to the low frequency signal, which is cyclic. In practice under the '207 patent, one correlates the tag capability for the generation of voltage at the low frequency with capacitance change of the third circuit element, and vice versa, to enhance the magnitude of the phase reversals across the third circuit element, which generate the sidebands of the reradiated signal.
In the '640 patent, the principle underlying the reradiator element is that of an un-symmetrical dipole, which is folded back to conserve length. In the '207 patent tag device, the narrow sections form part of the radiating RF element of a symmetrical dipole. Again, as in the '640 patent, part of the pattern is folded back to conserve space.
In a copending and commonly-assigned application, Ser. No. 562,749 entitled "Electronic Article Surveillance System and Tag", there is provided a tag which incorporates a reradiator which is configured as a monopole. A monopole antenna typically requires only half as much length as a dipole and encompasses a ground plane to that effect. In customary monopole configurations, the ground plane is required to be perpendicular to the reradiator element of the monopole and of considerable size. This is because monopole radiator elements are of length normally near one-quarter wavelength and operate at or close to their natural resonance. Per the invention of the referenced copending application, however, the reradiator element has considerable inductive reactance and a large ground plane is neither required nor desirable. The resonant matching condition thus is controlled by impedances of the components of the monopole, such as its diode and a spiral reradiator element.
In the preferred embodiment of the invention of the referenced application, a tag uses a reradiator element which comprises a spirally wound inductor, which can be both very short and narrow without much loss of efficiency. The ground plane used is a reasonably narrow and short strip of conductive material and placed in line with the spiral element. By choosing a diode with suitable impedance characteristics, the limited size of the in-line ground plane can be made an integral part of the overall impedance matching system.
A significant and valuable feature of the invention of the referenced application is that all of the components are short, to conserve length, and narrow, to conserve width. Thus, a very compact tag design is achieved in accordance with that invention with performance comparable with existing larger tags.